Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

Explorer 33 was designed to become the first U.S. spacecraft to enter lunar orbit (planned parameters were 1,300 x 6,440 kilometers at 175° inclination), but the Thor Delta E-1 second stage accelerated too rapidly for compensation by the probe's retrorocket to achieve lunar orbit.

Instead, the spacecraft (56.7 kg by this time) went into an eccentric Earth orbit of 15,897 x 435,330 kilometers. The main solid-propellant retrorocket engine later stabilized the orbit to a less eccentric 30,550 x 449,174-kilometer orbit at 28.9° inclination. In its new orbit, the probe returned key data on Earth's magnetic tail, the interplanetary magnetic field, and radiation.

Key Dates

1 Jul 1966: Launch (16:02:25 UT)

21 Sep 1971: Contact Lost

Status: Partial Success

Fast Facts

This was intended to be the United State's first lunar orbiter, but it was travelling too fast to be caputred by the Moon's gravity.

NASA's Lunar Orbiter 1 achieved that goal later in 1966, snapping hte first picture of Earth from the Moon (right).

Even though Explorer 33 missed its primary target, it still returned useful information on interplanetary space.